Such systems of control usually employ magnetic or optical transducers or encoders for resolving the scale divisions of either linear or rotary scales. Linear scales are usually fixed in positions paralleling and spanning the axis of freedom, and, the scale reader or encoder rides on the carriage in that axis in a position to read the scale. Rotary scales are usually driven by the screw which drives the carriage in the axis or by the drive motor therefor.
Two encoders have usually been employed in resolving such scales. U.S. Pat. No. 2,848,698 describes an incremental position detector comprising a magnetic scale and two magnetic heads in quadrature spaced relationship with respect to the grooves in the scale. The grooves define the scale divisions. These magnetic heads individually produce time varying voltages. These voltages are in quadrature phase relationship. The voltages are logically processed to produce square or rectangular quadrature phased voltages which are useful in determining carriage position, velocity and direction of movement in the axis of freedom. U.S. Pat. No. 3,245,144 which describes a machine tool control, with particular reference to FIGS. 19, 137, and 143 and the related text, describes a system for logically processing the quadrature phase voltages derived from magnetic heads scanning a linear scale in a machine axis, for controlling movement and position of a machine tool carriage in that axis. U.S. Pat. No. 3,262,105 applies such principals to a rotary scale, adding the provision of an index mark defining the start of the rotary scale. The system uses the index signal or voltage derived from the index mark and the scale division count thereafter for defining carriage position and/or velocity and direction of movement.
All of the systems of these patents employ two magnetic heads in resolving the linear or rotary scales. All of them employ or are intended for use with systems of logic responsive to the instantaneous electrical states of the signals or voltages of the magnetic heads for determining the direction of movement in the axis, and for determining position by counting scale divisions, using the voltage state relationship of the two signals at particular intervals of time. When two magnetic heads are employed in quadrature phase position relationship along the scale, one or the other of the heads will always change electrical state when movement occurs. When the electrical outputs of both magnetic heads are simultaneously high, the voltage of one will drop depending upon which direction the magnetic head assembly has been moved. If both magnetic heads are in their low voltage states, the voltage of one magnetic head will go high, depending on the direction of movement. When one magnetic head is in the higher of its two voltage states and the other in the lower voltage state, the start of movement in one direction will cause the magnetic head in the higher of its two voltages to switch to the lower of its two voltage states while the voltage of the other magnetic head in the lower of its two voltage states remains unchanged. When movement starts in the reverse direction the magnetic head in the lower of its two voltage states switches to its higher voltage state while the magnetic head in the higher voltage state remains in the higher of its two voltage states.
The logical processing of these signals may be accomplished by complementing the output signals so that the output signals and their compliments exist simultaneously. Next, pulses are produced by differentiating the output signals and their compliments. Thereafter the pulses are selectively combined with the output signals and the complemented output signals to produce 4 count pulses per scale division in separate count pulse sets for each direction of displacement. By such an expedient, the system measures position and determines the direction of movement. The rate of count pulse production is of course an indication of the rate of movement.
With such arrangements when a single scale is used, the two heads must be precisely positioned to include one quarter of a scale division spacing. Where two scales are employed, one for each head either the heads or the scales may be displaced to include one quarter of a scale division of spacing. It is apparent that the use of multiple heads and/or scales adds to the cost of the incremental encoder. The additional parts in such an encoder arrangement increases the probability of failure. The logical processing of these quadrature signals, such as discussed above, including differentiating of the signals and the complementing of the encoder produced signals, together with the necessary circuits for logically combining these signals, adds to cost and to the parts count and the likelihood of failure.